I have been looking at Color Contrasting double stars and comparing their color differences to the differences in their spectral class (by subtype). When I plot these differences there is a reasonable correlation as seen below. One of my data points is the optical pair S639 in Virgo which is a clear outlier looking at my plot and I suspect the spectral class of the magnitude 10 secondary which is listed as K0 in Simbad is in error. Looking at the color index of the secondary and my photo I suspect that it perhaps is at least a late F or early G star. Sky Safari lists the spectra of the primary as M0 III which does help a bit. Any chance those of you here on this forum with spectrographs could take a look at these two stars?
Edited by ssmith, 06 June 2020 - 01:01 PM.